Oxy-Fuel Combustion Combined Cycles for Carbon Capture

Doctoral thesis, 2015

A short and medium term method to decrease carbon dioxide emissions is carbon capture and storage. This method captures carbon dioxide from point sources of emissions and then stores the carbon dioxide in geological formations. The aim of this thesis is to analyse and compare two different types of combined cycles that are well suited for carbon capture and storage. The cycles are the Graz cycle and the Semi Closed Oxy-fuel Combustion Cycle (SCOC-CC). The power output of the cycles analysed here is around 100MW, which is in the mid-size power output range. The two cycles are compared to a conventional cycle that has a net efficiency of 56%. Two different layouts of the Graz cycle have been compared in this thesis. The first is a more advanced layout that incorporates a second bottoming cycle, which utilizes the heat of condensation from the flue gas condenser. The second layout is a simplified version of the Graz cycle that does not incorporate the second bottoming cycle, and is as such more comparable to the layout of both a conventional combined cycle and the SCOC-CC. The more advanced Graz cycle has around 48% net efficiency, while the simplified Graz cycle and the SCOC-CC has around 46.2% efficiency. Another aim was to develop tools that are able to design the gas turbines that are used in oxy-fuel combustion cycles. The combustion products are mainly steam and carbon dioxide. This influences the properties of the working media in the gas turbines used in the cycles. Traditional design tools for the gas turbine therefore need modification. The thesis describes the conceptual design tool used to design the compressor part of the gas turbines. The tool is based on a one dimensional model that uses empirical data to compute losses. The thesis also describes the development of a two dimensional compressor design method. The Graz cycle has a high water content while the SCOC-CC has a high carbon dioxide content. This difference in the working fluid will result in the turbomachinery being smaller for the Graz cycle compared to the SCOC-CC. A twin-shaft gas turbine was concluded to be better suited than a one shaft for the two oxy-fuel combustion cycles. However, the first stage of the power turbine needs to be cooled.